<!DOCTYPE html>
<html lang="en">

<head>
     
    <meta charset="UTF-8">
     
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>Document</title>
     
     
    <link rel="stylesheet"href="https://cdn.jsdelivr.net/npm/bootstrap@3.3.7/dist/css/bootstrap.min.css"      
         integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u"      
         crossorigin="anonymous">
     <style>
        body {
            height: 200vh;
        }

        .main {
            background-color: red;
            width: 300px;
            height: 300px;
            position: fixed;
            left: 0;
            top: 200px
        }

        .back {
            background-color: aqua;
            width: 300px;
            height: 300px;
        }
    </style>
</head>

<body>
    <div class="container-fluid">
        <div class="back"></div>
    </div>
</body>

</html>